The present invention relates to a semiconductor device and, more particularly, to a semiconductor device suited for a high-frequency amplifier.
A power amplifier module using an InGaP/GaAs heterojunction bipolar transistor is widely used in the fields of cell phones and the like.
The reliability of a device can be improved by using an InGaP layer as an emitter material instead of an AlGaAs layer.
The efficiency of a power amplifier is an important characteristic which extends a continuous conversation time. One effective method of increasing the efficiency of a power amplifier is to reduce an offset voltage Vce offset. To reduce the offset voltage Vce offset, it is possible to use a double heterojunction bipolar transistor using a wide-gap material in a collector layer as well as in an emitter layer.
If a thick InGaP layer is used as a collector layer, however, the mobility of electrons in this InGaP layer becomes lower than that in a GaAs layer. Therefore, if the collector layer is entirely replaced with an InGaP layer, the high-frequency characteristics significantly deteriorate.
Accordingly, to reduce the offset voltage Vce offset, as shown in FIG. 13, it is effective to form a thin InGaP layer 3 only in a portion of a collector layer in contact with a GaAs base layer 2, and form the rest of the collector layer by a GaAs layer 4.
In this structure, however, if the InGaP layer 3 is ordered, “+” interface electric charge is produced in the interface between the InGaP collector layer 3 and GaAs collector layer 4. FIG. 14 shows a bandgap diagram in this case. This interface electric charge deteriorates the distortion characteristics of the amplifier. This will be explained in detail below.
FIG. 15 shows the output characteristics, i.e., the dependence of a collector current IC on an emitter-to-collector voltage VCE of a single heterojunction bipolar transistor (SHBT) using a wide-gap material only in an emitter layer, and a double heterojunction bipolar transistor (DHBT) using a wide-gap material in an emitter and collector as shown in FIG. 13.
Referring to FIG. 15, a curve L1 indicates the characteristic of the SHBT when a base current IB is 0.2 mA, a curve L2 indicates the characteristic of the DHBT when the base current IB is 0.2 mA, a curve L3 indicates the characteristic of the SHBT when the base current IB is 0.1 mA, and a curve L4 indicates the characteristic of the DHBT when the base current IB is 0.1 mA.
FIG. 15 shows that the offset voltage Vce offset of the DHBT is lower than that of the SHBT. Therefore, an increase in efficiency of an amplifier using this DHBT can be expected. In reality, however, the efficiency cannot be increased because the distortion characteristics of the amplifier deteriorate for the reasons explained below.
FIG. 16 shows changes in base-to-collector capacitance Cbc with respect to a collector-to-base voltage VCB. Referring to FIG. 16, a curve L11 indicates a capacitance when a forward bias is applied in the base-to-collector path of the DHBT, and a curve L12 indicates a capacitance when a forward bias is applied in the base-to-collector path of the SHBT.
As shown in FIG. 16, the increase in capacitance when a forward bias is applied in the base-to-collector path in the DHBT is larger than that in the SHBT.
Referring to FIG. 17, a line L21 indicates a change in maximum available gain with respect to a collector-to-base voltage VCB in the SHBT, and a line L22 indicates a change in maximum available gain with respect to the collector-to-base voltage VCB in the DHBT. The increase in capacitance when a forward bias is applied in the base-to-collector path makes the decrease in maximum available gain when a forward bias is applied in the base-to-collector path in the DHBT larger than that in the SHBT. As a consequence, the distortion characteristics of the amplifier deteriorate.
If an unordered InGaP layer is used as the collector layer, the generation of the interface electric charge as described above can be suppressed. However, band discontinuity in the conduction band in the junction portion between the p-type GaAs base layer 2 and n-type InGaP collector layer 3 blocks electrons injected from the base layer 2 into the collector layer 3. FIG. 18 shows a band diagram in this case. As shown in FIG. 18, at a point P11, a barrier is formed by band discontinuity in the conduction band in the junction portion between the p-type GaAs base layer 2 and n-type InGaP collector layer 3, and this barrier blocks the flow of electrons.
References disclosing the conventional semiconductor devices for high-frequency amplifiers are as follows.    Patent reference 1: Japanese Patent Laid-Open No. 2001-345328    Patent reference 2: Japanese Patent Laid-Open No. 2001-176881    Patent reference 3: Japanese Patent Laid-Open No. 2002-134524    Patent reference 4: U.S. Pat. No. 5,952,672    Patent reference 5: U.S. Pat. No. 6,465,816.
As described above, the conventional semiconductor devices have the problem that when the offset voltage Vce offset is decreased in order to increase the efficiency, the distortion characteristics deteriorate by the influence of interface electric charge generated between an ordered InGaP layer and a GaAs layer in a collector layer.